It is well known that polyester resins, particularly polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate resins have excellent mechanical, physical and chemical performances and, therefore, are widely utilized for fibers, films and other shaped articles.
Among these polyesters, polyethylene terephthalate resin is usually produced by a two step process. Namely, in the first step, an ethylene glycol ester of terephthalic acid and/or a polymer with a low degree of polymerization produced by a method in which terephthalic acid and ethylene glycol are directly subjected to a esterification reaction; or a method in which a lower alkyl ester of terephthalic acid, for example, dimethyl terephthalate, and ethylene glycol are subjected to a transesterification reaction, or a method in which terephthalic acid and ethylene oxide are reacted with each other; and then in a second step, the reaction product of the first step is subjected to a polycondensation reaction in the presence of a polymerization catalyst, while heating the reaction mixture under a reduced pressure with the polymerization degree of the resultant product reached a desired level, to produce the target polyethylene terephthalate.
It is well known that, in the polyester resin produced by the above-mentioned process, the reaction rate and the quality of the resultant polyester resin greatly depend on the type of the catalyst used in the polycondensation step. With respect to the above-mentioned dependency, it is also known from the previous research results that when a catalyst comprising an antimony compound or a germanium compound which has been widely used as a polycondensation catalyst, is used, the polycondensation can be carried out at an excellent efficiency and the resultant polyester resin exhibits a good color tone.
However, when continuous melt spinning of a polyester produced by using an antimony compound as a polycondensation catalyst is carried out over a long period of time, there arises the problems that foreign matter adheres and is deposited around a spinneret for melt spinning. This foreign matter may be referred to as spinneret foreign matter hereinafter. The deposited foreign matter causes a bending phenomenon of the molten polyester streams extruded from the spinneret, which leads to the decrease in formability of the polymer melt and the occurrence of fuzz and/or breakage of filament yarns obtained in the melt-spinning and drawing steps.
Generally, as catalysts for the polyester usable for PET bottles, etc., germanium compounds are used. In this connection, there are problems that germanium is a rare metal and is expensive and, thus, the resultant resin products are also expensive.
There has been proposed to use, as other polycondensation catalyst than the antimony compounds and the germanium compounds, titanium compounds, for example, titanium tetrabutoxide. When titanium compounds are used, the above-mentioned problems on the formability of the polyester melt, derived from the deposition of the spinneret foreign matter, can be solved. However, another problems, that the resultant polyester resin is colored yellowish and exhibits an insufficient thermal stability of the polyester melt, occur.
For example, Japanese Examined Patent Publications No. 48-2229 (Patent Reference 1) and No. 47-26597 (Patent Reference 2) disclose that, as titanium compounds other than those mentioned above, titanium hydroxide or α-titanic acid is used as a catalyst for the production of polyesters. However, when titanium hydroxide is used, it is not easy to pulverize titanium hydroxide, and when α-titanic acid is used, α-titanic acid has a low stability of the chemical structure thereof and, thus, is difficult to preserve and handle. Therefore, these titanium components are not appropriate to use for industrial practice, and hardly produce a polyester polymer having a good color (b value).
To solve the above-mentioned problems, there have been attempted to use a product obtained by a reaction of a titanium compound with a specific phosphorus compound as disclosed in WO 01/00706 (Patent Reference 3) and WO 03/008479 (Patent Reference 4), and a non-reacted mixture or a reaction compound of a titanium component with a specific phosphorus compound as disclosed in WO 03/027166 (Patent Reference 5), as a catalyst for the production of the polyester polymer. It is true that the above-mentioned catalysts contribute to enhancing the thermal stability of the polyester melt and to improving the color of the resultant polymer. However, these catalyst cause the reaction rate in the production of the polyester to be low, and thus a problem, that the productivity of the polyester is somewhat low, occurs.
For the purpose of enhancing the stable formability of the polyester the method in which no antimony is used is an effective means. However, the no-antimony method causes the color of the resultant polymer and polymer products to be unsatisfactory and thus has not been utilized in practice. Therefore, a polyester produced by using a catalyst containing no antimony and having a good color is required.
In the production of the above-mentioned polyester, usually, the polymerization temperature must be controlled to approximately from 280 to 300° C. However, it is known that this high polymerization temperature causes a problem that the resultant polymer is discolored due to thermal decomposition of the polymer, namely, the polyester per se is colored yellow.
For the purpose of solving the yellow-coloring problem as mentioned above, usually, a cobalt compound is mixed into the polyester polymer to decrease the yellow-coloring, as disclosed in, for example, Japanese Unexamined Patent Publication No. 51-128397 (Patent Reference 6). The addition of the cobalt compound can certainly improve the color (b value) of the polyester polymer. However, the addition of the cobalt compound causes a plurality of problems in that, regarding the thermal stability of the melt of the polyester polymer, the added cobalt compound deposits in the polymer and causes foreign matter to be generated, and the processability of the polyester polymer, to produce a desired shaped article, and the quality of the resultant shaped article are probably affected. Further, in the case where a manganese compound is used as a catalyst for the transesterification reaction, and a cobalt compound is added, as a tinting agent, into the reaction mixture, the resultant polyester polymer has the problems that the cloths and fibers made from the resultant polyester polymer may be discolored when treated with a bleaching agent.
Also, as an attempt of improving the color of the polyester polymer, a polyethylene naphthalate resin knead-mixed with a dye is disclosed in, for example, Japanese Unexamined Patent Publication No. 11-158257 (Patent Reference 7), Japanese Unexamined Patent Publication No. 3-231918 (Patent Reference 8), Japanese Unexamined Patent Publication No. 11-158257 (Patent Reference 9) and Japanese Unexamined Patent Publication No. 11-158361 (Patent Reference 10).
Reference List
Patent Reference 1 JP-48-2229-B
Patent Reference 2 JP-47-26597-B
Patent Reference 3 WO 01/00706 pamphlet
Patent Reference 4 WO 03/008479 pamphlet
Patent Reference 5 WO 03/027166 pamphlet
Patent Reference 6 JP-51-128397-A
Patent Reference 7 JP-11-158257-A
Patent Reference 8 JP-3-231918-A
Patent Reference 9 JP-11-158257-A
Patent Reference 10 JP-11-158361-A